1. Field of the Invention
This invention relates to a brushless motor drive circuit.
2. Background Prior Art
A motor drive circuit is generally classified as a current drive type or a voltage drive type. Normally, a brushless motor drive circuit of the voltage drive type is automatically braked by inactivating the supplied rotary command. On the other hand, however, a brushless motor drive circuit of the current drive type cannot be braked by inactivating the rotary command and requires a brake circuit to be added to the drive circuit.
FIG. 9 of the accompanying drawings shows a known brushless motor drive circuit of the current drive type comprising a brake circuit (disclosed in Japanese Patent Disclosure No. 58-172992).
The disclosed brushless motor drive circuit has a group of switching devices 1 and a brake circuit 3. The group of switching devices 1 comprises switching devices 12a and 12b. The switching device 12a is constituted by transistors Q.sub.10 through Q.sub.12 and Q.sub.20 through Q.sub.22. The switching device 12b is constituted by transistors Q.sub.13 through Q.sub.15 and Q.sub.23 through Q.sub.25 and a resistor R.sub.1. The brake circuit 3 comprises transistors Q.sub.30 through Q.sub.32 and is so designed to respectively connect their collectors to the bases of the transistors Q.sub.13 through Q.sub.15 so that a brake signal is applied to each of the transistors Q.sub.13 through Q.sub.15.
For starting the motor, select signal Sda is supplied to each of the bases of the transistors Q.sub.10 through Q.sub.12 to turn off the transistors Q.sub.20 through Q.sub.23 and, at the same time, select signal Sdb is supplied to each of the bases of the transistors Q.sub.13 through Q.sub.15 to turn on the transistors Q.sub.23 through Q.sub.25. Then, the rotor of the motor is driven to rotate as the current supplied to the drive coils Lu, Lv and Lw is sequentially switched.
For stopping the rotary movement of the rotor, the select signals Sda and Sdb are cut off to turn off all the transistors Q.sub.20 through Q.sub.25 of the switching devices 12a and 12b. Then, brake signal Bs is applied to each of the bases of the transistors Q.sub.30 through Q.sub.32 of the brake circuit 3. This causes the transistors Q.sub.23 through Q.sub.25 of the switching device 12b to be turned on once again. Consequently, the counter electromotive voltages generated in the drive coils Lu, Lv and Lw are brought into a short-circuit condition to brake the drive circuit.
With such a conventional brushless motor drive circuit as described above, the potentials of the middle points C of the drive coils Lu, Lv and Lw are lowered as low as the ground potential when the transistors Q.sub.23 through Q.sub.25 are turned on so that consequently the potentials of the edges U, V and W of the drive coils Lu, Lv and Lw become lower than the ground potential because of the negative counter electromotive voltages involved. Moreover, the reactive current which is used for turning on the switching device 12b and is significant in any sense, remains alive after the rotor is stopped.